The present invention relates to a process for the continuous, solvent- and mastication-free production of pressure-sensitive self-adhesive compositions based on non-thermoplastic elastomers, using tackifier resins, typical rubber plasticizers, optionally fillers and heat-activatable crosslinkers, and to the coating thereof to produce self-adhesive articles, in particular high-performance self-adhesive tapes.
Fundamental to adhesive systems and the pressure-sensitive adhesive articles produced with them are the two physical phenomena of adhesion and cohesion. Adhesion is dealt with in the technical jargon using the terms instant bond strength (tack) and bond strength (peel strength) and describes by definition the terms xe2x80x9cself-adhesivexe2x80x9d and/or xe2x80x9cpressure-sensitive adhesivexe2x80x9d, i.e. permanent adhesive bonding under xe2x80x9cgentle pressurexe2x80x9d.
Especially in the case of pressure-sensitive adhesives based on natural rubber, this property is obtained by mixing in tackifying resins (tackifiers) and plasticizers having relatively low molecular weights.
The second defining property of the pressure-sensitive adhesives is their simple residue-free redetachability after use. This property is determined essentially by the high molecular mass rubber fractions as the elastomer component, which give the system, in the form of cohesion (internal strength), the required strength under shear stress, which is of particular significance for the use of the products at relatively high temperatures and/or mechanical loads. By additional crosslinking, for example by way of ionizing rays, reactive resin components or other chemical crosslinkers, this property can be reinforced.
The performance of the pressure-sensitive adhesive is, therefore, critically determined by the balanced proportion of adhesion properties and cohesion properties and by compatibility, homogeneity and stability of the blend of components with extremely high and relatively low average molecular weights, something which is relatively easy to achieve in the case of production of the composition in industry-standard mixers and kneading machines using solvents.
The solvent-free compounding and processing of self-adhesive compositions, on the other hand, has become established primarily only for the processing of elastomers which melt, so-called thermoplastic elastomers.
In this case, the process of producing the composition is normally conducted in the melt in twin-screw extruders at relatively high temperatures, with coating taking place normally by means of slot dies.
The advantage of using thermoplastic elastomers lies essentially in a simplification of the coating process. The avoidance of flammable solvents does away with the need for the drier units, with their high energy consumption for the evaporation and recovery of the solvents, and to use explosion-protected units. Hot-melt coating units are compact and permit much higher coating speeds. Moreover, the technology is an environment-friendly one in which there are no solvent emissions.
For the solvent-free compounding of thermoplastic elastomers, the prior art makes use predominantly of block copolymers having polystyrene block fractions. The advantage of this class of substance is that the polystyrene domains present in the polymer soften above 100xc2x0 C., accompanied by a sharp fall in the viscosity of the adhesive composition and thereby providing ease of processing. After cooling to room temperature, the polystyrene domains are reformed and impart a certain shear strength to the pressure-sensitive adhesives based on thermoplastic elastomers.
The thermoplastic elastomers can be compounded faultlessly in the extruder process using hydrocarbon resins which promote bond strength. In this way, a desired level of bond strength can be achieved with relative ease. The resultant pressure-sensitive adhesives, however, remain sensitive to temperatures above 40xc2x0 C. For the self-adhesive tapes produced on this basis, this remanent xe2x80x9ccreep behaviourxe2x80x9d is critical for unrestricted storage stability (blocking of the rolls in the stack, especially in the course of transportation in relatively warm climate zones) and for their use at relatively high operating temperatures (for example as masking tapes in automotive finishing, where despite postcrosslinking such tapes lose their functional capacity: the pressure-sensitive adhesive softens and the shear strength for fixing the masking papers is no longer ensured).
For this reason, the known hot-melt pressure-sensitive adhesives based on block copolymers have been able to establish themselves almost exclusively for packaging tapes and for labels for use at room temperatures.
Using non-thermoplastic elastomers, such as natural rubber, on the other hand, it is possible to achieve the required shear strengths; however, the solvent-free production and processing of natural-rubber pressure-sensitive adhesives has to date confronted the person skilled in the art with unsolved problems.
Owing to the extremely high molecular mass fractions of the rubber (with Mwxe2x89xa71 million), solvent-free self-adhesive compositions cannot be processed by the hot-melt pressure-sensitive adhesive technology, or else the rubbers used must be reduced in their molecular weight (broken down) so greatly before processing that as a result of this breakdown their suitability for high-performance self-adhesive compositions is impaired.
The deliberate industrial process of rubber breakdown under the combined action of shear stress, temperature and atmospheric oxygen is referred to in the technical literature as mastication and is generally carried out in the presence of chemical auxiliaries, which are known from the technical literature as masticating agents or peptizers, or, more rarely, as xe2x80x9cchemical plasticizing aidsxe2x80x9d.
In rubber technology, the mastication step is necessary in order to make it easier to integrate the additives.
According to Rompp (Rompp Lexikon Chemiexe2x80x94Version 1.5, Stuttgart/New York: Georg Thieme Verlag 1998), mastication is a term used in rubber technology for the breaking down of long-chain rubber molecules to increase the plasticity and/or reduce the (Mooney) viscosity of rubbers. Mastication is carried out by treating, in particular, natural rubber in kneading apparatus or between rolls at very low temperatures in the presence of mastication aids (masticating auxiliaries). The high mechanical forces which act lead to a xe2x80x9ctearing apartxe2x80x9d of the rubber molecules, with the formation of macroradicals, whose recombination is prevented by reaction with atmospheric oxygen. Mastication aids such as aromatic or heterocyclic mercaptans and their zinc salts or disulphides accelerate the mastication process by promoting the formation of primary radicals. Activators such as metal (iron, copper, cobalt) salts of tetraazaporphyrins or phthalocyanines enable the mastication temperature to be lowered. For the mastication of natural rubber, mastication aids are used in amounts of from about 0.1 to 0.5% by weight in the form of masterbatches, which facilitate a uniform distribution of this small amount of chemicals within the rubber composition.
Mastication must be clearly distinguished from the breakdown known as degradation which results in all of the standard solvent-free polymer technologies, such as compounding, conveying and coating in the melt.
Degradation is a collective term for various processes which change the appearance and properties of plastics. Degradation can, for example, be caused by chemical, thermal, oxidative, mechanical or biological influences or also by the effect of rays (such as (uv) light). Examples of consequences are oxidation, chain cleavages, depolymerization, crosslinking or separation of side groups of the polymers. The stability of the polymers with respect to degradation can be increased using additives, for example by adding stabilizers such as antioxidants or photostabilizers.
Uncontrolled degradation often constitutes an unwanted phenomenon. It can be minimized by providing an inert gas atmosphere.
A variety of routes to the solvent-free production and processing of rubber pressure-sensitive adhesives have been described.
The patent CA 698 518 describes a process for achieving production of a composition by adding high proportions of plasticizer and/or simultaneously strong mastication of the rubber. Although this process can be used to obtain pressure-sensitive adhesives having an extremely high tack, user-compatible shear strength, even with a relatively high level of subsequent crosslinking, can be achieved only to a limited extent owing to the relatively high plasticizer content or else to the severe breakdown in molecular structure of the elastomer to a molecular weight average of Mwxe2x89xa61 million.
The use of polymer blends, where besides non-thermoplastic natural rubber use is also made of block copolymers, in a ratio of approximately 1:1, is essentially an unsatisfactory compromise solution, since it results neither in high shear strengths when the self-adhesive tapes are used at relatively high temperatures nor in significant improvements relative to the properties described in the patent.
JP 07 324 182 A2 describes a multistage process in which a double-sided adhesive tape has a pressure-sensitive adhesive layer based on an acrylic resin adhesive and has a second layer comprising a blend of isoprene-styrene elastomer, natural rubber and non-reactive hydrocarbon resin (Arkon P 100). This tape is used as a carpet-laying tape, where there are likewise no stringent requirements on the shear strength at elevated temperatures.
The use of non-thermoplastic elastomers is also described in JP 95 331 197, where use is made of an isocyanate-reactive natural rubber (polyisoprene grafted with maleic ester) having an average molecular weight Mwxe2x89xa61 million with aliphatic non-reactive hydrocarbon resins, which is crosslinked with blocked isocyanates (for example Desmodur CT); the mixture is initially crosslinked at 150xc2x0 C. for five minutes and following its subsequent coating onto PET film is cured at 180xc2x0 C. for several minutes (for example 15 minutes). This procedure clearly shows how complicated it is to achieve postcrosslinking if the natural rubber is subjected to excessive breakdown during the production process.
The patent application JP 95 278 509 discloses a self-adhesive tape in which the natural rubber is masticated to an average molecular weight of Mw=100,000 to 500,000 in order to obtain a coatable homogeneous mixture comprising hydrocarbon resins, rosin/rosin-derivative resins and terpene resins, which can be processed readily at between 140xc2x0 C. and 200xc2x0 C. with a coating viscosity of from 10 to 50xc3x97103 cps but require an extremely high subsequent EBC dose (40 Mrad) in order to ensure the shear strength necessary for their use.
For carrier materials such as impregnated and/or sized papers, and for woven carriers based on viscose staple and the like, the system is not very suitable, since at the necessarily high beam doses there is significant carrier deterioration.
The use of exclusively non-thermoplastic rubbers as the elastomer component in the formulation of pressure-sensitive adhesives with the existing cost advantage possessed by, for example, natural rubbers over the standard commercial block copolymers, and the outstanding properties, especially the shear strength of natural rubber and of corresponding synthetic rubbers, is also set out at length in the patents WO 94 11 175 A1, WO 95 25 774 A1, WO 97 07 963 Al and, correspondingly, U.S. Pat. No. 5,539,033 and U.S. Pat. No. 5,550,175.
In these cases, the additives customary in pressure-sensitive adhesive technology, such as tackifier resins, plasticizers and fillers, are described.
The production process disclosed in each case is based on a twin-screw extruder which permits compounding to a homogeneous pressure-sensitive adhesive blend with the chosen process regime, involving mastication of the rubber and subsequent gradual addition of the individual additives with an appropriate temperature regime.
The mastication step of the rubber, which precedes the actual production process, is described at length. It is necessary and characteristic of the process chosen, since with the technology chosen therein it is indispensable to the subsequent integration of the other components and to the extrudability of the blended composition. Also described is the feeding in of atmospheric oxygen, as recommended by R. Brzoskowski, J. L. and B. Kalvani in Kunststoffe 80 (8), (1990), p. 922 ff., in order to accelerate mastication of the rubber.
This procedure makes it absolutely necessary to practise the subsequent step of electron beam crosslinking (EBC) and to use reactive substances as EBC promoters in order to achieve an effective crosslinking yield.
Both process steps are described in the abovementioned patents, but the EBC promoters chosen also tend towards unwanted chemical crosslinking reactions at elevated temperatures, which limits the use of certain tackifier resins.
Owing to the unavoidable high product temperatures, compounding in a twin-screw extruder prevents the use of heat-activatable substances suitable for crosslinking the adhesive compositions, such as, for example, reactive (optionally halogenated) phenolic resins, sulphur or sulphur-donor crosslinker systems, since the chemical crosslinking reactions which ensue in the process result in such a great increase in viscosity that the coatability of the resulting pressure-sensitive adhesive composition is impaired.
In summary, it can be stated that all of the known processes are characterized by extremely severe rubber breakdown. When the compositions are processed further into self-adhesive tapes, this necessitates extreme crosslinking conditions and results, moreover, in a partially restricted applications profile, especially as regards the use of resultant self-adhesive tapes at elevated temperatures.
There is numerous known apparatus for the continuous production and processing of solvent-free polymer systems. In common use are screw machines such as single-screw and twin-screw extruders in a variety of process lengths and with a variety of fittings. However, continuously operating kneading apparatus of a very wide variety of constructions, including, for example, combinations of kneading and screw machines, or else planetary roll extruders, are also used for this purpose.
Planetary roll extruders have been known for a fairly long time and were first used in the processing of thermoplastics such as PVC, for example, where they were used primarily to supply the downstream units such as, for example, calenders or roll mills. As a consequence of their advantage of the great renewal of surface area for material exchange and heat exchange, by means of which the frictional energy can be dissipated rapidly and effectively, and because of the low residence time and the narrow residence-time spectrum, their use in recent times has been extended, inter alia, to compounding processes which require a particular temperature-controlled regime.
Depending on manufacturer, planetary roll extruders are available in various designs and sizes. The diameters of the roll cylinders, depending on the desired throughput, are typically between 70 mm and 400 mm.
Planetary roll extruders generally have a filling section and a compounding section.
The filling section is comprised of a conveying screw to which all of the solid components are fed continuously. The conveying screw then passes the material to the compounding section. The area of the filling section, together with the screw, is preferably cooled in order to prevent baking-on of materials on the screw. Alternatively, there are designs without a screw section, where the material is fed directly between central spindles and planetary spindles. However, this is not important for the effectiveness of the process of the invention.
The compounding section is comprised of a driven central spindle and a number of planetary spindles which rotate around the central spindle within a roll cylinder with internal helical gearing. The rotary speed of the central spindle and hence the rotational speed of the planetary spindles can be varied and are therefore an importance parameter for the control of the compounding process.
The materials are circulated between the central and planetary spindles, or between the planetary spindles and the helical gearing of the roll section, so that under the influence of shear energy and external heating the materials are dispersed to form a homogeneous compound.
The number of planetary spindles rotating in each roll cylinder can be selected and thus adapted to the requirements of the process. The number of spindles influences the free volume within the planetary roll extruder, and the residence time of the material in the process, and also determines the surface area for heat and material exchange. By way of the shear energy introduced, the number of planetary spindles has an influence on the result of compounding. Given a constant diameter of roll cylinder, a larger number of spindles permits better homogenization and dispersion or, respectively, a greater product throughput.
The maximum number of planetary spindles installable between the central spindle and the roll cylinder depends on the diameter of the roll cylinder and on the diameter of the planetary spindles used. When using relatively large roll diameters, as required for obtaining production-scale throughputs, and/or relatively small diameters for the planetary spindles, the roll cylinders can be equipped with a relatively large number of planetary spindles. With a roll diameter of D=70 mm, typically up to seven planetary spindles are used, whereas with a roll diameter of D=200 mm ten, for example, and with a roll diameter of D=400 mm 24 for example, planetary spindles can be used.
In this context, reference is made to the patent applications and, respectively, utility model DE 196 31 182 A1, DE 94 21 955 U1, DE 195 34 813 A1, DE 195 18 255 A1 and DE 44 33 487 A1, which give an overview of the prior art in the field of planetary roll extruders.
Thus, furthermore, DE 39 08 415 A1 describes the processing of rubber mixtures or rubberlike mixtures by means of planetary roll extruders. For the purpose of further processing on downstream equipment, pre-batches or finished mixtures are masticated and plasticated on a planetary roll extruder. Likewise described is the production of finished mixtures in a planetary roll extruder: in this case vulcanizing systems and other components are metered into the rubber premixes.
DE 297 10 235 U1 discloses an apparatus for plasticating polymer material, said apparatus comprising at least two planetary roll extruders in parallel arrangement. The planetary roll extruders feed a common discharge stage, which for its part may be a single-screw extruder, a twin-screw extruder or a gear pump. Between the planetary roll extruders and the discharge stage it is also possible for there to be a devolatilizing unit, preferably comprising a vacuum-loaded drop shaft.
U.S. Pat. No. 3,825,236, as well, discloses the use of a planetary roll extruder, this extruder being located within a single-screw extruder. DE 23 03 366 A1 produces an extrudable composition comprising thermoplastic or thermosetting polymer in screw extruders with planetary rolls, the polymer, which is present in the form of granules or as a powder, being stuffed in the region of the planetary rolls and then masticated and plasticated, and compacted up to extrusion pressure. Claimed as essential to the invention in this case is that the stuffed polymer is precompacted up to the formation of solid bodies, then comminuted in the intake region of the planetary rolls, with pressure reduction, and also masticated, plasticated and compacted up to extrusion pressure.
The object of the present invention is to provide a process with which pressure-sensitive self-adhesive compositions based on non-thermoplastic elastomers can be produced continuously without solvent, with or without the use of thermally reactive components, and, if desired, can be coated in-line without the need for property-impairing mastication of the rubber.
This object is achieved by a process as set out in the main claim. The dependent claims relate to advantageous developments of the process. Finally, the concept of the invention also embraces self-adhesive tapes produced by the process of the invention.
The invention accordingly provides a process for the continuous solvent-free and mastication-free production of self-adhesive compositions based on non-thermoplastic elastomers in a continuously operating apparatus having a filling section and a compounding section, comprising the following steps:
a) feeding the solid components of the self-adhesive composition, such as elastomers and resins, into the filling section of the apparatus, optionally feeding of fillers, colorants and/or crosslinkers,
b) transferring the solid components of the self-adhesive composition from the filling section to the compounding section,
c) adding the liquid components of the self-adhesive composition, such as plasticizers, crosslinkers and/or further tackifier resins, optionally in the melted state, to the compounding section,
d) preparing a homogeneous self-adhesive composition in the compounding section,
e) discharging the self-adhesive composition and
f) coating the self-adhesive composition on a web-form material, where the coating of the web-form material is carried out using a multi-roll applicator unit, preferably using a 2- to 5-roll applicator unit, particularly preferably using a 4-roll applicator unit, so that the self-adhesive composition is shaped to the desired thickness as it passes through one or more roll nips, and where the rolls of the applicator unit can be individually set to temperatures from 20xc2x0 C. to 150xc2x0 C.
It has been found particularly advantageous to use, as the continuously operating apparatus, a planetary roll extruder whose compounding section is comprised preferably of at least two, but with particular preference three, coupled roll cylinders, it being possible for each roll cylinder to have one or more separate temperature control circuits.
Unlike otherwise conventional production processes, in the planetary roll extruder in accordance with the process of the present invention, in particular, there is no property-impairing mastication of the non-thermoplastic elastomers, since in this case they are not subjected separately to the effect of high shear energy but instead are always processed together with one or more liquid components. These liquid components can include both plasticizers such as oils, for example, and also resins which only melt during the compounding process under the action of shear energy and/or external heating. The presence of these liquid components limits the extent of frictional energy such that it is possible to avoid the mastication of the rubber, i.e. the molecular weight breakdown of the elastomers, and high resultant compounding temperatures.
Furthermore, planetary roll extruders have extremely large areas where material exchange and surface renewal take place, by which means it is possible to dissipate rapidly the frictional shear energy and thus to avoid undesirably high product temperatures.
The filling section of the roll cylinder is comprised of a conveying screw to which all of the solid components are fed continuously. The conveying screw then passes the material to the compounding section. The area of the filling section, together with the screw, is preferably cooled in order to prevent baking-on of materials on the screw. Alternatively, there are designs without a screw section, where the material is fed directly between central spindles and planetary spindles. However, this is not important for the effectiveness of the process of the invention.
As already stated, the number of planetary spindles has an effect on the compounding result by way of the shear energy introduced: given a constant roll cylinder diameter, a larger number of spindles can be used to achieve better homogenization and dispersion or a greater product throughput. In accordance with the present invention, preferably at least half, with particular preference at least ⅓, of the possible number of planetary spindles are to be used in order to obtain a good ratio of compounding quality to product rate.
For the process of the invention it is advantageous to use a planetary roll extruder whose compounding section has been extended by coupling at least two roll cylinders. Firstly, despite the presence of friction-reducing components for the purpose of avoiding mastication of the rubber, the complete digestion of the elastomer components, and the desired homogenization and dispersion at economic throughput rates, is possible by this means; secondly, the coupling of, preferably, separately temperature-controlled roll cylinders permits a balanced temperature regime of the process, so allowing the use of heat-activatable crosslinker systems.
Whereas in the front compounding section of the planetary roll extruder the roll cylinders are advantageously heated at temperatures above the melting point of the resins used, the rear compounding section is advantageously cooled in order to reduce the product temperature. By this means, the residence time of the self-adhesive composition at relatively high temperatures is kept as low as possible, thereby preventing the activation of the thermal crosslinker systems present in the self-adhesive composition.
Of course, any roll cylinder can be equipped differently in terms of the number and nature of the planetary spindles and so can be adapted to the particular formulation and processing requirements.
Between two coupled roll cylinders there is generally an approach ring through whose free cross section the central spindle is guided and which holds the planetary spindles of a roll cylinder in fixed location. Approach rings can have various free cross sections, thereby allowing the holdup of the product, and thus the filling level and/or extent of shear energy, to be varied and to be adapted to the requirements of the process. In addition, the approach rings can be provided with radial bores through which fluids such as, for example, plasticizer oils or else inert gases such as nitrogen, argon, carbon dioxide or the like can be supplied to the compounding section of the planetary roll extruder.
The central spindle, and each roll cylinder as well, should preferably have one or more separate temperature control or cooling circuits in order to allow a temperature regime which permits the use of heat-activatable crosslinking systems. In cases where this is unnecessary, the temperature control circuits of coupled roll cylinders can also be combined with one another in order to minimize the number of temperature control devices.
For the purposes of the invention, the filling section of the planetary roll extruder, and the central spindle, should be preferably not heated but instead cooled, in order to prevent baking-on of material on the filling screw and to ensure effective heat exchange with the adhesive composition.
In the process according to the present invention, all solid components, such as elastomers, fillers and adjuvants, resins, ageing inhibitors, etc., are metered together into the filling area of the planetary roll extruder. These substances can be added either as separate components in each case or else as a conjoint premix or else as partial premixes to the compounding apparatus. Metering in the components in the form of a premix is particularly appropriate if the components are in similar supply forms or have similar bulk densities, so that in this way the number of metering systems can be minimized. Premixes can be produced in a simple manner, for example, in powder mixers. Metering of the individual solid components takes place suitably by means of volumetric and gravimetric metering systems in customary designs. A further possibility is to add the fluid components, or only some of them, such as plasticizer oil, for example, to a premix.
The materials metered in are transported by the screw of the filling section into the first roll cylinder of the planetary roll extruder. Between each roll cylinder, fluid acomponents such as, for example, plasticizer oils, soft resins or resin melts can be added via bores in the approach rings. The amount of liquid added prior to the influence of shear energy can be used to influence both the degree of breakdown of the elastomers and the compounding temperature of the pressure-sensitive adhesive composition. For example, a particularly low level of molecular weight breakdown of the elastomers is obtained if a liquid plasticizer is added at a point in time when there has still been no influence whatsoever of frictional energy, i.e. when this plasticizer is either added to a solids premix or the plasticizer oil is metered in continuously between the filling screw and the first roll cylinder. Also possible is the subdivision of the liquid components, in the form of a so-called split feed, over the barrel length, and this is another parameter for controlling the process in terms of elastomer breakdown and product temperature.
The process of the invention permits the production of high-performance self-adhesive compositions and, especially in conjunction with a downstream coating and crosslinking unit, permits the production of high-performance self-adhesive tapes accompanied by particular cost advantages.
The process is comprised essentially of the above process steps, which can be carried out optionally under an inert gas atmosphere in order to avoid oxidative polymer degradation.
In the first process step, a composition comprising the elastomers and the known adjuvants required for the production of self-adhesive compositions, such as fillers, ageing inhibitors, plasticizers and tackifier resins, is produced solvent-free in a planetary roll extruder, the composition having a final temperature of less than 150xc2x0 C., preferably less than 130xc2x0 C. and, with very particular preference, between 70xc2x0 C. and 110xc2x0 C. The overal residence time of the composition in the planetary roll extruder should not exceed three minutes. The resulting hot-melt adhesive composition has a viscosity of between 20,000 Pa*s and 190,000 Pa*s, in particular a viscosity of between 45,000 and 120,000 Pa*s at 110xc2x0 C. under a shear rate of 0.1 rad/s.
In accordance with the invention, it is further proposed to carry out the coating of the web-form material solventlessly using a multi-roll applicator unit. This can be done in a particularly effective and advantageous manner using a 2- to 5-roll applicator unit, especially using a 4-roll applicator unit, so that the self-adhesive composition is shaped to the desired thickness as it passes through one or more roll nips prior to its transfer onto the web-form material.
The rolls of the applicator unit may be set individually to temperatures of from 20xc2x0 C. to 150xc2x0 C.
The preferred 4-roll applicator unit is formed of a metering roll, a coating-bar roll, which determines the thickness of the layer on the carrier material and is arranged parallel to the metering roll, and a transfer roll, which is located below the metering roll. On the layup roll, which together with the transfer roll forms a second roll nip, the composition and the web-form material are brought together.
This coating process is preferred when the viscosity of the self-adhesive composition exceeds levels of 5000 Pa*s under a shear rate of 1 rad/s, since then coating with extrusion dies no longer gives the required accuracy in the amount of composition applied.
Depending on the nature of the web-form carrier material to be coated, coating may take place by a co-directional or counter-directional process.
In one particularly advantageous embodiment of the invention, the rolls of the applicator unit may be set individually to temperatures from 70xc2x0 C. to 120xc2x0 C., in particular 90xc2x0 C. and 100xc2x0 C.
In order to generate a sufficiently precise, thin shaped adhesive film, the peripheral speeds of the rolls have differences.
For instance, without wishing to restrict the concept of the invention, the differential speeds in the case of an appropriate 4-roll applicator unit may be as follows:
It has further been found that the coating process of the invention may be carried out with particular advantage by means of a 4-roll applicator unit whose rolls have the following properties:
According to DIN 53505 (1987-06) the Shore hardness in the testing of elastomers and rubber corresponds to the resistance to the penetration of a conical frustum (A or C) or of a rounded-off cone (D), which is measured by the compression of a spring having defined spring characteristics and is expressed in dimensionless Shore A(C, D) hardness units. In the case of the testing of steel, the Shore rebound hardness is measured in the so-called scleroscope, which determines the rebound height of a drop hammer which drops onto the test surface within a vertical tube (Rxc3x6mpp Lexikon Chemiexe2x80x94Version 1.5, Stuttgart/New York: Georg Thieme Verlag 1998).
Depending on the viscosity of the self-adhesive composition, various processes are suitable for coating the web-form materials. Self-adhesive compositions having viscosities of up to 5000 Pa*s under a shear rate of 1 rad/s, as are obtained, for example, using relatively high proportions of plasticizer oils or by adding thermoplastic elastomers to the non-thermoplastic elastomers, can be coated by means of an extrusion die downstream of the planetary roll extruder, the preferred extrusion die used being a coathanger manifold die. In order to obtain a defined, full-area application of composition on the web-form material, it is advantageous for the self-adhesive composition to be subjected to devolatilization before entering the coating die, this being particularly important in the case where inert gases are used during the compounding process in the planetary roll extruder.
In accordance with the process of the present invention, the devolatilization takes place under the action of reduced pressure, preferably in screw machines which are at the same time able to overcome the pressure drops of the pipelines and coating die. For this purpose, particular preference is given to single-screw conveying extruders with downstream distributor die which in addition have a pressure regulation system, so that the coating of the web-form carrier materials can take place with amounts of applied composition with a very low range of fluctuation.
The distributor die should, based on a cross section of the coating-bar roll, be positioned in the region from 9 to 12 o""clock, preferably in the region between 10 and 11 o""clock, in order to achieve appropriate coating results.
Coating on roller coating applicator units or multi-roll coating calenders is possible at or temperatures below 100xc2x0 C., so that even self-adhesive compositions containing heat-activatable crosslinkers can be coated. For the purpose of increased freedom from gas bubbles in the coated adhesive composition, it is possible to install a vacuum devolatizer between the planetary roll extruder and the applicator unit; for example, a vacuum chamber, a devolatilizing extruder or the like.
Advantageously in conjunction with production of the composition and coating, in a third process step, the self-adhesive composition is crosslinked and specifically by the coating of the web-form material being carried out using the multi-roll applicator unit, where the shaped adhesive composition is crosslinked on the last non-web-guiding roll (in particular the transfer roll) with the aid of electron beams or UV rays by means of ionizing radiation, such as electron beams, for example, so that the resultant self- adhesive tape becomes shear-resistant and temperature-stable. UV rays as well can be used for crosslinking, in which case appropriate UV promoters, for example such as Ebecryl 140 from UCB, must be added to the self-adhesive composition.
For further improved performance, or in the case of EBC-sensitive carriers, crosslinking can also be carried out by means of heat-activatable crosslinkers under the effect of temperature.
The heating of the pressure-sensitive hot-melt adhesive composition that is required for this purpose can be done with the aid of the known techniques, especially with the aid of high-temperature ducts, or else with the aid of infrared lamps, or by means of high-frequency magnetic alternating fields, examples being HF waves, UHF waves or microwaves.
Crosslinking of the pressure-sensitive hot-melt adhesive composition can be carried out, furthermore, by means of a combination of ionizing radiation and heat-activatable chemical crosslinkers.
The result is a highly shear-resistant pressure-sensitive self-adhesive composition whose properties are comparable with those of similar self-adhesive compositions produced in a solvent process.
With the process of the invention, virtually without exception, all known components, described in the literature, of rubber-based self-adhesive compositions can be processed without solvent.
Advantageously, the non-thermoplastic elastomer is chosen from the group of the natural rubbers or of the synthetic rubbers or consists of any desired blend of natural rubbers and/or synthetic rubbers, it being possible to choose the natural rubber or rubbers in principle from all available grades, such as, for example, crepe, RSS, ADS, TSR or CV grades, depending on the required purity and viscosity level, and it being possible to choose the synthetic rubber or rubbers from the group of the randomly copolymerized styrene-butadiene rubbers (SBR), butadiene rubbers (BR), synthetic polyisoprenes (IR), butyl rubbers (IIR), halogenated butyl rubbers (XIIR), acrylate rubbers (ACM), ethylene vinyl acetate copolymers (EVA) and polyurethanes, and/or blends thereof.
With further preference, thermoplastic elastomers with a weight fraction of from 10 to 50% by weight, based on the overall elastomer content, can be added to the non-thermoplastic elastomers in order to improve their processability.
As representatives, mention may be made at this point in particular of the highly compatible styrene-isoprene-styrene (SIS) and styrene-butadiene-styrene (SBS) grades.
Tackifier resins which can be used are, without exception, all tackifier resins known to date and described in the literature. Representatives that may be mentioned include the rosins, their disproportionated, hydrogenated, polymerized and esterified derivatives and salts, the aliphatic and aromatic hydrocarbon resins, terpene resins and terpene-phenolic resins. Any desired combinations of these and other resins can be used in order to adjust the properties of the resultant adhesive composition in accordance with what is desired. Reference may be made expressly to the depiction of the state of knowledge in xe2x80x9cHandbook of Pressure Sensitive Adhesive Technologyxe2x80x9d by Donatas Satas (van Nostrand, 1989).
Plasticizers which can be used are all plasticizing substances known from adhesive tape technology. They include, inter alia, the paraffinic and naphthenic oils, (functionalized) oligomers such as oligobutadienes and oligoisoprenes, liquid nitrile rubbers, liquid terpene resins, animal and vegetable oils and fats, phthalates, and functionalized acrylates.
For the purpose of heat-induced chemical crosslinking, it is possible in the process of the invention to use all known heat-activatable chemical crosslinkers, such as accelerated sulphur or sulphur donor systems, isocyanate systems, reactive melamine resins, formaldehyde resins and (optionally halogenated) phenol-formaldehyde resins and/or reactive phenolic resins or diisocyanate crosslinking systems with the corresponding activators, epoxidized polyester resins and acrylate resins, and combinations thereof.
The crosslinkers are preferably activated at temperatures above 50xc2x0 C., in particular at temperatures from 100xc2x0 C. to 160xc2x0 C. and, with very particular preference, at temperatures from 110xc2x0 C. to 140xc2x0 C.
The thermal excitation of the crosslinkers can also be effected by means of IR rays or high-energy alternating fields.
Further embraced by the concept of the invention is a self-adhesive tape produced with the aid of the pressure-sensitive hot-melt adhesive composition by applying the self-adhesive composition to at least one side of a material in web form. Depending on the intended use of the adhesive tape, suitable web-form carrier materials for the self-adhesive compositions processed and produced in accordance with the invention are all known carriers, with or without appropriate chemical or physical surface pretreatment of the coating side, and anti-adhesive physical treatment or coating of the reverse side. Mention may be made, for example, of creped and non-creped papers, polyethylene, polypropylene and mono- or biaxially oriented polypropylene films, polyester, PVC and other films, foam materials in web form, made from polyethylene and polyurethane, for example, wovens, knits and nonwovens.
Finally, the web-form material can be a material with an antiadhesive coating on both sides, such as release papers or release films.
The thickness of the self-adhesive composition on the web-form material can be between 10 xcexcm and 2000 xcexcm, preferably between 15 xcexcm and 150 xcexcm.
Finally, the self-adhesive composition can be applied in a thickness of from 800 xcexcm to 1200 xcexcm on a release paper. An adhesive-composition layer of this kind, especially after crosslinking, can be used diversely as a backingless double-sided self-adhesive tape.
Owing to the high molecular weight of the unmasticated elastomers, the process of the invention provides for the first time the possibility of being able either to do away entirely with crosslinking the pressure-sensitive adhesive composition or else to carry out effective crosslinking which permits, for example, high-temperature applications of the adhesive tapes produced by this process, by means of high-energy radiation, without the need for promoters for this purpose. Furthermore, as a result of the temperature regime at a lower level, it is now also possible for the first time to use heat-activatable crosslinkers in the production of solvent-free self-adhesive compositions on the basis of non-thermoplastic elastomers.
The self-adhesive compositions produced in accordance with the process of the invention are highly shear-resistant. The rubbers used are not subjected to mastication, but in the individual process steps degradation processes occur which do not, however, durably impair the properties of the self-adhesive composition. Furthermore, these processes are restricted and are manageable as a result of the process of the invention.
The invention is described in more detail below with reference to the following examples, without wishing thereby to restrict the invention.
The test methods used are briefly characterized as follows:
The bond strength (peel strength) of the compositions was determined in accordance with AFERA 4001.
The shear strength of the adhesive compositions investigated was determined in accordance with PSTC 7 (holding power). All values given were determined at room temperature under the stated loads of 10 or 20 N, with a bond area of 20xc3x9713 mm2. The results are reported in minutes of holding time.
In the examples, a planetary roll extruder from the company ENTEX Rust and Mitschke was used. The diameter of a roll cylinder was 70 mm, its length being varied between 400 mm and 1200 mm.